An electric motor is used in a hybrid car in combination with an engine or in an electric car which is driven only by an electric motor. When the electric motor is driven, an inverter is used to obtain desired torque and frequency. The inverter is built in a car and is demanded to achieve high power at high integration density to ensure a passenger space.
Depending on driving environments of a car, an operation temperature of the inverter greatly changes. Particularly in a hybrid car, the inverter has a high temperature under influence of heat generation of an engine. Switching elements in the inverter have a risk that the temperature increases under influence of constant loss due to a current through the switching elements themselves and switching loss due to switching on/off, in addition to influence of such an environmental temperature, and a breakdown may occur when a certain temperature is exceeded.
To avoid breakdown of the switching elements, the temperatures of the switching elements need to be detected for protection. Since temperature detection can be performed with high accuracy by using an on-chip sensor in which a diode is assembled in an element, such a sensor is mounted on an inverter for a hybrid car. Since the on-chip sensor has a high electric potential, temperature information needs to be transferred to a micro-controller (hereinafter abbreviated as mi-con) through an insulating circuit. In an inverter on which at least six switching elements are mounted, a plurality of insulating circuits are required to detect temperatures of all the switching elements. A board area increases and may cause an inverter capacity to increase.
To solve problems as described above, a method can be considered in which temperature detection is performed only partially on several of the switching elements and temperatures of the other switching elements are estimated. There has been disclosed a method in which one or more switching element temperatures are detected and a temperature of a switching element having a maximum temperature is estimated from thermal transfer models of IGBT modules.
According to a conventional method, loss models of switching elements and thermal transfer models belong to a steady state. In transition to a state in which a current concentrates only on a part of the switching elements, e.g., when a motor is locked or rotates at a low frequency, a switching element which has a maximum temperature can not steadily be specified, and switching element temperatures can not accurately be estimated, either. In addition, thermal transfer models are constructed on the basis of estimated temperature values of switching elements existing around a switching element which is estimated to have the maximum temperature. Therefore, when a basing switching element temperature includes an error, an estimation error concerning the switching element temperature indicating the maximum temperature increases.
The embodiments have been made to solve problems as described above, and have an object of reducing the number of components forming an inverter apparatus and reducing a board space by accurately estimating temperatures of the other switching elements, based on a temperature detected from one switching element, whatever state an operation mode of the inverter is in.